This invention relates generally to so-called high torque fasteners particularly of the type described in U.S. Pat. Nos. 2,677,985 and 3,540,342. More specifically, this invention relates to an improved high torque fastener and an improved driving tool therefor, wherein the fastener and tool are designed for facilitated and positive engagement and rotational alignment of the tool with the fastener.
Threaded fasteners in general are well known to include a threaded shank joined integrally at one end with a screwhead shaped for engagement by an appropriate driving tool. For example, fasteners, such as screws, are used widely wherein the screw has a slotted head for reception of a blade or blades of a driving tool, such as a standard blade or Phillips-type screwdriver or the like. The specific configurations of the fastener head and driving tool can vary widely, particularly with fasteners designed for conventional use requiring relatively low levels of driving torque for installation. However, in a few specialized environments, such as a variety of military and aerospace applications, the fastener head and driving tool are carefully geometried to accommodate relatively high driving torques during installation thereby achieving higher tensile preloads or fastening forces. Such specialized fasteners are referred to commonly as "high torque" fasteners.
One particularly well-known high torque fastener design is shown and described in U.S. Pat. No. 2,677,985 wherein a fastener head is provided with a generally bow-tie shaped driving slot defined by an arcuately concave bottom wall separating slightly undercut side walls. This driving slot is adapted for reception and engagement by a driving tool blade having an arcuately convex lower surface generally matching the curvature of the slot bottom wall. Upon rotation of the driving tool, the undercut geometry of the slot side walls cooperates with the driving tool blade to accommodate relatively high torque transfer between the driving tool and the fastener head.
While the fastener slot and tool blade geometry described in said U.S. Pat. No. 2,677,985 has proven particularly successful in accommodating the desired high torque driving forces, major difficulties are encountered particularly with respect to initial engagement and rotational alignment between the relatively blunt arcuate surface of the driving tool blade and the relatively shallow slot in the fastener head. Such initial engagement and rotational alignment is especially difficult with relatively small fasteners or in situations where the fastener is not readily visible to the tool operator, such as when the tool operator is required to reach over or around other structures to install the fastener at a blind position. Improper engagement and noncentered alignment between the driving tool and the fastener head upon initial tool rotation can result in lateral movement or "walking" of the tool across the fastener head and adjacent structural surfaces to damage or otherwise scar those surfaces, particularly when the driving tool is power driven for relatively high speed rotation.
In U.S. Pat. No. 3,540,342, a modified high torque fastener is shown and described intended to reduce incidences of driving tool blade "walking." This modified fastener includes a head having a relatively shallow dimple formed centrally with respect to the driving slot, wherein this dimple has a diameter substantially greater than the slot width and a depth substantially less than the slot depth. This recessed dimple is intended to assist in initial engagement of the driving tool blade centrally relative to the fastener head, thereby to constrain the blade against "walking" upon initial rotation thereof and for seating into the driving slot. While this modified fastener geometry offers some improvements in initial tool-fastener engagement, careful operator attention is still required to insure proper engagement between the tool and fastener head, particularly with fasteners of small sizes. These engagement difficulties are compounded by the blunt shape of the tool blade and the shallow depth of the alignment dimple whereby it is often difficult for the tool operator to recognize alignment by "feel" as opposed to visual inspection. Moreover, the blunt shape of the driving tool blade permits engagement with the blade and fastener rotationally misaligned relative to each other, in which case the driving tool blade tends undesirably to disengage from the fastener and "walk" over adjacent structural surfaces upon initial tool rotation. Further, noncentered rotational misalignment between the tool blade and the fastener results in unbalanced application of driving forces to the fastener head which, in some instances, can result in damage to the fastener.
There exists, therefore, a significant need for an improved high torque fastener and associated driving tool designed for relatively rapid and positive fastener-tool engagement and rotational alignment. The present invention fulfills this need and provides further related advantages.